The reason you most likely aren't ranking in the UK is you don't appear to have any company address on the site (in the UK or other wise), but the site is hosted on a US IP address. So if the only clue to your location is your IP address, Google will use that. So if Google is thinking your site is in the US, there really isn't much reason for it to rank it in a UK based search.
The usual thing to suggest to get you ranking in the UK would be to Geo target your domain to the UK, but would you want to sacrifice your ranking in .com? As I'm pretty sure this would effect your .com ranking.
So to keep your ranking in .com and get ranking in UK searches, one approach would be to create an version of your site for the UK - like you have currently have for your Chinese, German, French and Japanese pages.
You could created a sub folder such as:
http://www.translationzone.com/uk/ (such as Apple do )
Add a version of the site in there to target the UK and you can Geo Target just that folder to the UK in Google Web Master Tools. Make sure to mark up the site using the correct rel="alternate" hreflang="x" language and locale attributes so Google knows its for the UK and how it relates to the rest of the site.
More reading can be done here on mult lingual sites guidelines.